A 6-Step Method for Mastering Structural Repair

Although we’re about to discuss holding and blocking, this article isn’t about last month’s Super Bowl or on defensive strategies for your Sunday backyard tag-football game. (Besides, I’ve seen you play, so if I were going to give you advice, it would be to retire.)

I am, however, here to help. But what I’m here to help with are the basics for properly repairing today’s unitized and conventional structured vehicles.

Too often today, there’s a lack of understanding when it comes to effectively repairing today’s vehicles. Even with all the educational courses being offered by I-CAR and the equipment manufacturers, key factors for effective repairs aren’t always being practiced.

To maintain consistency in repair quality, structural repairs require the following six basic steps for success:

1. Analyzing and developing a repair plan through measurement.
2. Holding the vehicle securely without causing additional damage.
3. Using blocking techniques and other tools to improve the repair process.
4. Monitoring pressure during the repair.
5. Relieving the stress to restore the state of the metal.
6. Validating repairs through measurement.

Too often we shortchange one of the above only to spend hours of additional time (or even worse — rework) figuring out what’s wrong. This reminds me of the Tortoise and the Hare story. Why jump and "just pull," believing you’ve gotten ahead? As the story goes, taking a methodical approach wins in the long run.

Step 1: Formulate a Plan (a.k.a. The Ol‘ Sittin‘ on a Milk Crate Theory)
Knowing exactly how to drive somewhere doesn’t always guarantee your actual arrival. In this case, having the latest and most advanced measuring system and a good plan doesn’t fix the vehicle — but it’s the best starting point.

Today’s vehicles absorb a great deal of energy from the impact. To do this requires the impact energy to be dissipated throughout the structure. But this creates challenges during the repair process since the impact point may be small, yet the energy expands dimensionally throughout the body.

This is just one reason it’s important to let the measuring system develop the pulling plan. Too often, secondary damage is missed when a technician just begins pulling without properly identifying all the damage first.

I’ve retold this story many times before, yet it’s still as apropos as it was when I first heard it. A veteran technician once asked me what was his most important tool. Being fairly green at structural repairs, I didn’t have an answer. He then pointed to a milk crate filled with all types of air and hand tools and said: "The milk crate." I didn’t understand at first until he turned it over, sat down and said, "It allows me to sit and think about what happened in the collision before clamping and pulling and ‘hoping’ it will work."

Along with having a nearby milk crate, you should also have the specifications and the current readings before pulling. These will provide you with the necessary information to make good decisions.

But what you need today aren’t just vehicle dimensional specifications. You also need vehicle design specifications. This includes knowing when a part should be repaired and when it shouldn’t be. From all this information, you can then develop a road map of what needs to be done and how it needs to happen.

Step 2: Hold It!
Holding is critical to properly repair a vehicle. And remember that whatever pulling force is placed on the vehicle is transmitted to the areas of anchoring, too. This is why the anchoring area must be secure enough to prevent vehicle movement but also to withstand the pulling force without distorting the anchor area. On most unitized structures, this involves the pinchweld area at the front and rear torque box areas. Since this area generally has some reinforcements and/or multiple layers of metal, it’s usually (but not always) the suggested anchoring location.

For most pulling systems, each pinchweld clamp has multiple smaller "jaws" attached to it. This divides the pulling force equally between each clamping jaw, provided each is properly tightened and mounted.

For example, let’s say you’re going to apply 2,000 pounds of force to pull the damage back into position. Each area where these clamps are mounted needs to resist 250 pounds of pressure without causing distortion. This is because each of the pinchweld jaws divides the force equally from the pulling process (2,000 divided by 8 jaws = 250). However, if only two clamping units are used or only one jaw is tightened properly on each clamp, the force doubles to 500 pounds at each clamp location. At this point, you’re getting close to causing damage.

It’s also important to define "properly" mounted or tightened. If pinchweld clamps aren’t properly tightened and the clamp jaws slip, then the force is directed to the other mounted clamp jaws. This is why you need to ensure the pinchweld clamps are free from anything that may interfere with a secure anchoring. Pinchweld clamp jaws should be inspected, cleaned regularly and replaced periodically when they become worn or damaged.

Some unibody vehicles either don’t have reinforced pinchweld areas or even a pinchweld. These vehicles have special anchor adapters designed for the pulling system or require modifications to the structure for anchoring. This can include welding or removing suspension parts to bolt the anchoring fixture. Regardless, you shouldn’t vary from these anchoring locations and just make your own.

So what should you do about trucks and SUVs? They may require special frame adapters or have special mounting requirements with special anchor sets as listed above. With many frame-type vehicles, a multiple-sized frame hole clamp can still be used — if used properly. But be careful. If you pull a clamp downward, it’ll use its leverage and act as a can opener. Keep the clamp horizontal! You can accomplish this many ways. One is to place blocks under the clamp so it can’t pull down at the mounting hole. Another is to place a small hydraulic ram on the clamp to keep it horizontal. No matter the method, keeping the clamp horizontal is the goal — even if the chain is angled downward 45 degrees.

It’s also important to hold an area in place after it’s restored to the proper dimensions and to not allow it to continue past or stretch past its proper position while the repair process continues. This is sometimes accomplished with holding fixtures. One common type of holding fixture is called a "turn buckle." This unit has small clamps on each end that can be mounted in a door opening and tightened to lessen the upper body from flexing. If you’ve ever pulled the front structure with the door open or off, you probably know what I’m talking about. Even with the doors closed, body flex is present.

Step 3: Go for the Block!
Seems like I’m constantly referring to sports — only to let you down by not actually discussing football. And here I go again! In this case, blocking is a method that allows the use of leverage to ensure the pulling force is concentrated to the proper area of damage. It also allows for the block to place force (normally upward) at a damaged area needing upward movement or to keep an area from moving downward during the pulling process of another area.

As with anchoring clamps, the force the block places on its location is determined by the amount of force placed against it during pulling. A block placed under a cross member or structural member will exert the same force upward as the pull is directing downward. This is really useful when restoring datum misalignment.

During pulling, some of the pulling force is absorbed in the body due to flexing of the unitized structure. The additional force necessary to create movement in the damaged area from the loss when the body flexes can cause pulling clamp areas to tear. An analogy of breaking a "green" tree branch verses a old one illustrates how the green branch requires more effort to break because it flexes before breaking. Blocking can also assist in lessening this flexing.

Step 4: There She Blows! Why to Pressure Monitor
It’s certainly been said hundreds of times that you should pull a unitized structure with a minimal amount of force. But how do you know how much force you’re applying if you aren’t using a gauge to monitor it? (And I don’t recommend counting the seconds between each pump beat.) You need to visually see what pressure each of your pulls is placing on the structure to know when your pulling plan is successful. And if you have to use excessive force — usually anything more than 2,500 to 3,000 pounds — your pulling plan probably isn’t correct. There may be times in very reinforced center section areas that you’ll reach this suggested limit, but I normally find it possible to stay below 2,500 pounds.

And many of today’s pulling systems have pressure gauges to make sure you do. If the pulling system uses foot hydraulic air pumps, these gauges have a tendency to get damaged since they’re often mounted at the pump base. Be careful with them. Other gauges are located on the frame rack itself and may be difficult to see during the pulling. But that’s no excuse. You still need to use them.

To help reduce the amount of force needed to pull today’s vehicles, identify anything that may be restricting the damage from moving, such as an engine cradle or cross member. For example, you may want to unbolt a front bolt of a damaged engine cradle to allow the rest of the structure to move. If the rear engine-cradle mounting area is out of specs, then add a pull to the cradle itself to pull that location while the other structure is being pulled. It may also help to cut away some of the primary damage to allow access to the rest of the structural damage. Keep in mind that using welded absorbers as pulling attachments normally doesn’t supply the proper pulling points and, many times, just pulls out the mounting bolts.

Step 5: Get Rid of the Stress
Once the vehicle is properly anchored and blocked and the clamps are mounted, the time has come to pull. But pulling — and continuing to pull — without relieving the internal metal’s state will likely cause the structure to crack or tear. This happens because the metal, whether high strength steel or mild steel, has been work hardening at the damage and will want to crack or tear before moving — unless you use the proper techniques. One of the proper techniques is to stress relieve the metal around the damage.

You can accomplish stress relieving with vibration (heat and/or mechanical). A misconception here has led many organizations to not recommend the use of heat. During stress relieving, heat is used to warm up the molecules to allow them to internally vibrate — and to move. We aren’t softening the steel as many of us did on older conventional frames to ease the pulling load. The heat to cause this vibration is very low — generally under 1,250 degrees and even much lower at times — so using the oxy/acetylene torch isn’t necessary. A small propane unit is all the heat you need for stress relieving. In fact, there’s nothing wrong with using an air heat gun with a top temperature of 750 - 1,000 degrees.

If you’re going to use this method, understand that many vehicles have specific time and temperature limits. Heat indicators and heat crayons are but a few examples of methods you can use to ensure that you don’t exceed temperature limits. The amount of time also may have specific limits. For these reasons, using heat is my least recommended method to accomplish stress relieving.

I’ve found that mechanically vibrating the metal around the damage is easy and the most effective method. And you can accomplish it with hammering spoons or pneumatic bits. The vibration quickly reduces the metal stress and lessens the pulling force needed. In fact, if a pull is held at 1,500 to 2,000 pounds and the pulling direction is correct, the pressure gauge should drop significantly during stress relieving. And this drop verifies your pulling plan.

Step 6: Validating Your Work
At this point, you need to verify that all the work has been done correctly and completely by re-measuring the vehicle. Today, this generally means a computer-aided measuring device, from which a print out can be made and kept with the file. You can also give a copy to the vehicle owner. What other way can you possibly verify the repairs if you ever have to "prove it"?

The liability issues surrounding today’s repairs are continuing to increase every day — so the equipment to validate repairs is no longer a novelty but, rather, standard for today’s collision repair center.

This validation step is too often skipped in many repair centers. Yet it’s critical. I’ve never been one to resist testing or proving what I do or say is correct. And I know many of you out there feel the same. To those of you who don’t, well ... don’t be surprised if, before long, your services are no longer needed.

The process of pulling to a reasonable pressure, holding, stress relieving and validating will achieve the most efficient pulling process with the lowest chance of tearing or cracking metal. These steps likely aren’t new to you, yet it’s also likely that you’ve seen many a repair done by someone else who didn’t use any of them.